NATURE 



[April 4, 191; 



Anv meteorolog"lcal condition which causes a 

 body of air to cover a wide stretch of sea without 

 any irrej^ular disturbance favours the development 

 of cloud-sheets of this type. Fig. i ^ shows an 

 example of a cloud-sheet which occurred at Brook- 

 lands on September 26, 1917, with a well-marked 

 south-westerly current. The clouds extended from 

 3000 ft. to 4000 ft., and there was a temperature 

 recovery of 6° F. in 200 ft. above them ; there 

 was some irregular disturbance near the ground, 

 but not sufficient to disturb the cloud-sheet, where 

 the eddies were evenly distributed. This type of 

 cloud-sheet is most common in quiet winter 

 weather, and the eddies are then not so well 

 marked as in Fig. i, but are, nevertheless, easily 

 seen, and cause the clouds to be called " strato- 

 cumulus." In anticyclones the air is very warm 

 above the clouds, and in winter the temperature 



northern France. Their effect in preventing noc- 

 turnal radiation is of great importance. 



The clouds of the cumulus class are caused by 

 turbulence on a much larger scale than that which 

 occurs in the horizontal cloud-sheets. They are 

 formed with strong winds or when the air is being 

 heated at the surface, and are commonest over 

 land and on summer days. , The winds crossing 

 the irregular surface of the land, or local differ- 

 ences of temperature, give rise to small variations 

 of pressure, which cause irregular vertical currents 

 and corresponding variations in the horizontal 

 wind velocity. The vertical currents do not usually 

 last long, and soon mix with the surrounding air, 

 but the turbulence extends upwards, and 

 thoroughly rtiixes the air up to the height of a 

 few thousand feet. The turbulence in these con- 

 ditions is more violent and less regular than that 



Fig. I. — Strato-cumulus at . 



Rise of temperature 6' F. in 200 ft. above clourl 



from 6000 ft. , Brooklands, 4.30 p.m., September 26, 1917, 



C. K. M. D. 

 High clouds of approaching " rain-line " above. Taken, 



recovery may amount to 15° F. in lopo ft., as on 

 December 22, 1917, and January 5, 1918, in 

 northern France. Cloud-sheets at these two dates 

 were almost certainly formed originally over the 

 sea; the first was at about 5000 ft., moving from 

 N.'N.E., the second at about 4000 ft.-, moving 

 from W.S.W. For fully 2000 ft. below the cloud- 

 sheets there was much turbulence and an adia- 

 batic lapse-rate, while within 1500 ft. of the 

 ground there was no turbulence noticeable to aero- 

 planes, and the lapse-rate was zero. Such ad- 

 vances of turbulent cloud-sheets from the sea over 

 the top of comparatively tranquil air near the 

 ground are common in winter in Britain and 



3 [We are indebted to the Scottish Meteorological Society for permission 

 to reproduce the three photographs illustrating Capt. Douglas's paper 

 They are selected from a beautiful series of thirteen, all taken by Capt. 

 Douglas, which accompany a paper by him on "The Lapse-line and its 

 Relation to Cloud Formation " in the last i.ssue of the Society's Journa 

 Third Series, vol. xvii.. No. 34).— Ed. Nature.] 



which causes horizontal cloud-sheets. The clouds 

 are due partly to mechanical mixing of layers of 

 different temperature, partly to the adiabatic ex- 

 pansion of ascending air. The form may be 

 cumulus, fracto-cumulus, or strato-cumuluS, and 

 the amount depends mainly on the humidity of the 

 layers mixed up. When the turbulence is due 

 mainly to the wind passing over obstacles on the 

 ground, the temperature of the top of the turbu- 

 lent region is reduced, and a temperature inversion 

 is often formed above the clouds. The irregular 

 disturbances cause the upper surface of the clouds 

 to be uneven, as in Fig. 2, where the variations of 

 the level of the tops of the clouds amounted to 

 1000 ft. The highest portions of these clouds 

 reached 8000 ft. , and the temperature recovery 

 above these portions amounted to 6° F. There 

 was already a thin, broken, horizontal cloud-sheet 



NO. 2527. VOL. lOl] 



